1) Field of the Invention
The present invention relates to a process for producing a light absorption layer of a solar cell which layer consists of a compound semiconductor of Group IB, Group 3A, and Group 6A of the periodic table. More specifically, the present invention relates to a process for producing a light absorption layer of a solar cell which layer consists of a ternary alloy comprising copper-indium-selenium.
2) Description of the Prior Art
A solar cell is a device to convert light energy into electrical energy and is normally composed of a light-transmitting electrode, a light absorption layer consisting of a photoelectric semiconductor, and an electrode laminated sequentially on an insulating substrate. As for the photoelectric semiconductor, it has been considered that the highest efficiency for photoelectric conversion is given by a thin layer of ternary alloy of copper-indium-selenium, the atomic ratios of which are 1:1:2. However, it has been difficult to control the thickness of the thin layer of the alloy while controlling the ratios of the three components.
That is, in the production of the light absorption layer by vapor deposition which has been widely employed for the formation of thin layers, it has been very difficult to control exactly the ratios of copper, indium, and selenium. In a process for alloying the vapor deposited layer after each component has been deposited, it has also been difficult to attain a uniform alloy. In addition, such alloying method tends to cause fluctuation of the composition in a heat treatment step and also has a problem of high production cost for forming a light absorption layer having a large area. Further, although thin films of copper, indium, or alloys thereof can be formed by electrolytic plating at a relatively low cost, electrodeposition of selenium is not easy.
Recently, a process for producing a light absorption layer of ternary alloy of copper-indium-selenium has been proposed, in which a copper-indium layer with dispersed selenium particles is formed by using an alloy electrodeposition bath of copper and indium, in which a dispersion of fine selenium particles is suspended, and the thus formed deposition layer is alloyed in such a manner as disclosed in PCT Publication No. WO 92/05586. However, according to this process, it is required to balance the conditions in the electrodeposition bath, such as pH, temperature, concentration of metal ions, current density, stirring speed, and the like in order to control the composition of the electrodeposited layer. Such control is relatively easy if the constituents are copper and indium; however, there is difficulty in controlling the amount of selenium particles contained in the deposit. Accordingly, it has been very difficult to produce the light absorption layer of the solar cell in a stable manner with good performance.
It is therefore an object of the present invention to eliminate the problems encountered with the prior art method and to provide an improved process for producing in a stable manner a light absorption layer of a solar cell which comprises a ternary alloy of copper-indium-selenium having an atomic ratio controlled to be in a desired narrow range.